Present invention reletes to a magnetic paint and a process for producing the same, and more specifically, it relates to a magnetic paint in which magnetic particles, in particular, those having an average particle diameter of not greater than 0.25 .mu.m can exhibit excellent dispersibility rapidly and easily upon preparing a magnetic paint, possess a great saturation magnetization and a high coercive force for a long period of time while preventing oxidation with oxygen in air as much as possible, and can be transported and stored with an industrial and economical advantage merely by vacuum-packaging with a synthetic resin film, as well as a process for producing such a magnetic paint.
In recent years, with progressing longer recording time as well as miniaturization and weight-reduction of magnetic recording and reproducing apparatuses for video and audio applications, a violent development of VTR (video tape recorders) have been conducted and a demand for improving a performance of magnetic tapes as a magnetic recording medium, for example, higher recording density and improved output characteristics.
The above-mentioned properties of the magnetic recording medium have a close relationship with magnetic particles used for the magnetic recording medium. In recent years, magnetic iron based alloy particles having a higher coercive force and a greater saturation magnetization as compared with conventional magnetic iron oxide particles have been noted and put to practical use in digital audio-tapes(DAT), 8 mm-width video tapes, Hi-8 tape and video floppies. Along with the demand for higher image quality of the video tapes, a frequency of carrier signals to be recorded has become higher, that is, has been shifted to a shorter wavelength region, and as a result, a magnetization depth from the surface of a magnetic tape becomes remarkably shallow as compared with the recording of conventional video tapes.
In view of the above, the improvement of high output characteristics to signals of short wavelength while maintaining a CN ratio has been conducted and it has been required for this purpose to 1 make the magnetic particles finer, 2 increase the dispersibility of the magnetic particles, 3 smooth the surface of the magnetic coating layer, and 4 reduce the film thickness of the magnetic coating layer.
The facts mentioned above are stated in, for example, Development for Magnetic Material and High Dispersion Technology of Magnetic Powder published from Kabushiki Kaisha Sogo Gijutsu Center, p. 312(1982) as "Condition required for high density recording in a coating-type tape is that high output characteristics can be maintained at a low noise level for short wavelength signals, and for this purpose, it is necessary that both of the coercive force (Hc) and the residual magnetic flux density (Br) are great, and the thickness of the coating layer is further reduced", as well as Nikkei Electronics, May 3,(1976) pp. 82-105 as "As physical quantities of a tape related to the SN ratio (CN ratio) of luminance signals, an average number of particles per unit volume, their state of dispersion (dispersibility), and the surface smoothness may be mentioned. Since the SN ratio is improved in proportion with the square root for the average number of particles providing that the surface property and the dispersibility are constant, magnetic particles with a smaller particle volume and higher packing density are more advantageous."
Also, the residual magnetic flux density (Br) of the magnetic recording medium depends on the dispersibility in a vehicle, the orientation property in a coated film and the packing property of magnetic particles.
Improvement for the characteristics of the magnetic particles has been demanded more and more, and in view of the improvement for the noise level of the magnetic recording medium and the enhancement for the output characteristics of the magnetic recording medium, it is required that the magnetic particles are fine particles, and have an excellent dispersibility in a magnetic paint, and in particular, it is required that magnetic iron based alloy particles, can possess a great saturation magnetization and a high coercive force for a long period of time by preventing oxidation with oxygen in air as much as possible.
It has been known that the noise level oft, he magnetic recording medium has a close relationship with the particle size of the magnetic particles used and that the noise level is tends to be more lowered and improved as the particles size becomes smaller. In particular, fine magnetic particles of not greater than 0.25 .mu.m in particle size have been demanded in recent years.
On the other hand, the size of the magnetic particles becomes smaller, the more the dispersibility of the magnetic particles in the magnetic paint is lowered and as a result, the smoothness at the surface of the resultant magnetic coating film is worsened, making it difficult to reduce the film thickness. In view of the above, it has been demanded for a magnetic paint comprising fine magnetic particles capable of providing excellent dispersibility rapidly and easily.
Heretofore, Japanese Patent Applications Laid Open (KOKAI) Nos. 62-22867, 64-79274 and 64-79265 disclose a magnetic paint prepared by kneading magnetic particles, a binder resin and an organic solvent by using a kneader having a high shearing force such as a kneader, planetary mixer, disperser and twin-shaft continuous kneader to form a kneading composition, and then diluting to the thus-obtained composition of an appropriate solid concentration by using a diluting kneader such as a planetary mixer, a disperser, a paint conditioner and a twin-shaft continuous kneader, prior to the dispersion of the composition composed of magentic particles, the binder resin and the organic solvent into a vihicle by using a disperser such as a ball mill or a sand grinder.
In a conventional method, it is difficult to obtain rapidly and easily an excellent dispersion upon producing the magnetic paint. As described in, for example, Magnetic Paint Considered from a Point of Research Worker for Magnetic Recording Material, in Powder Metallurgy, "Seventh Summer Ferrite Seminar--Summary of Lecture" (1977), pp. 14-16, "Generally, it is a rather difficult to obtain high dispersion even in the case of a usual non-magnetic paint. Much more, in the case of the magnetic powder, the phenomenon is made more complicate due to the magnetic interaction between the particles"; the magnetic particles form a coagulated form due to magnetic interaction between each of particles and accordingly, the particles are present in coagulated form in the vehicle. Particularly, in the case of plate-like ferrite particles having an axis of easy magnetization in perpendicular to the plate surface, since the particles are coagulated to each other due to the magnetic interaction, it is difficult to separate the coagulated particles into individual particles merely by a mechanical treatment.
As magnetic particles which are suitable to high output and high density recording, that is, as magnetic particles having high coercive force, magnetic iron based alloy particles have been known.
As the magnetic iron based alloy particles, there can be mentioned magnetite (FeO.sub.x Fe.sub.2 O.sub.3 :0&lt;x.ltoreq.1) particles having characteristics of a higher coercive force, a greater saturation magnetization and less chargeability when used in a magnetic recording medium due to high electroconductivity as compared with those of maghemite particles not containing ferrous, as well as magnetic iron oxide particles obtained by using the above-mentioned magnetite particles or the meghemite particles described above as precursor particles and depositing cobalt and ferrous to the surface of the precursor particles (hereinafter referred to as Co-coated magnetic iron oxide particles).
Although the magnetic iron based alloy particles as described above possess a high coercive force, they involve a drawback of magnetic and chemical instability, and it has been pointed out a problem in view of the operability or safety upon preparing the magnetic particles, as well as a problem from a safety and economical point of view in transportation and storage.
That is, when the magnetic iron based alloy particles are left in air, ferrous is oxidized into ferric iron and as a result the magnetic property, in particular, the coercive force and saturation magnetization is reduced with the passage of time. The phenomenon tends to be remarkable as the particle size becomes smaller.
Further, the magnetic iron based alloy particles have been generally obtained by using, as the starting material, acicular iron (III) oxide hydroxide particles, acicular hematite particles obtained by dehydrating the acicular iron (III) oxide hydroxide particles under heating at temperature of less than 300.degree. C., or high-density acicular hematite particles obtained by heat-treating the acicular iron (Ill) oxide hydroxide particles in a non-reducing atmosphere at a temperature of not more than 300.degree. C., and reducing such a starting material under heating in a hydrogen gas. The formation of oxide layers to the surface of the thus-obtained particles is carried out by various kinds of well-known methods such as supplying method of an inert gas in which an oxygen content therein is increased gradually, whereby the thus-obtained magnetic iron based alloy particles are stabilized against oxidation with oxygen in air and then can be taken out into air.
However, even magnetic iron based alloy particles having the oxide layer formed at the surface of the particles, can not be effectively kept from oxidation with oxygen in air only by the oxide layer after being taken out into air, so that the saturation magnetization and the coercive force is gradually reduced with passage of time and, further, there is a danger of generating heat or causing ignition during transportation or storage.
In particular, the coercive force of the magnetic iron based alloy particles is improved as the particle size is reduced. However, since the surface activity of the particles becomes excessively large and the particles violently react with oxygen in air to heat-generate and, in an extreme case, to cause ignition, thereby lowering the coercive force and the saturation magnetization, an utmost care is necessary for the handling.
Further, the magnetic iron based alloy particles liable to cause danger such as of heat-generation or ignition are transported and stored, for example, by a method of sufficiently wetting the magnetic iron based alloy particles with an organic solvent and tightly sealing them in a container made of metal such as a can, or a method of placing the magnetic iron based alloy particles in a metal container such as a can and then tightly sealing them after purging with an inert gas. However, there are a safety problem due to the use of the organic solvent and operational and economical problems such as in packaging, handing and in the space for transportation and storage.
In view of the above, there is demanded a method for transporting or storing magnetic iron based alloy particles safely with the least danger and with an economical advantage capable of minimizing the space for the transportation and storage by a simple package.
It has been most demanded at present that the magnetic iron based alloy particles are fine particles, have an excellent dispersibility in a magnetic paint, can maintain a high coercive force and a large saturation magnetization for a long period of time while preventing oxidation with oxygen in air as much as possible, and can be transported and stored with an economical and industrial advantage by package which is safe without danger, convenient and simple. But, there has not yet been supplied a method capable of satisfying such various demands.
Namely, magnetic iron based alloy particles obtained by the well-known method inevitably suffer from remarkable reduction of the saturation magnetization and the coercive force due to the oxidation with oxygen in air with the passage of time when they are taken into air, if the particles are fine, in particular, not greater than 0.25 .mu.m. In addition, upon preparing the magnetic paint, dispersibility of the particles in the magnetic paint is not yet sufficient, because the effect of the shearing stresses to the magnetic iron based alloy particles during kneading is not sufficient.
In addition, in the case of using the known method for transportation and storage, there are safety problems due to the use of the organic solvent, as well as industrial and economical problems such as in packaging, handing in the space for transportation and storage.
It has, accordingly, been demanded for a provision of a magnetic paint in which the dispersibility of fine magnetic iron based alloy particles having an average particle diameter of not greater than 0.25 .mu.m in the magnetic paint is excellent, and large saturation magnetization and high coercive force can be maintained over a long period of time by preventing oxidation due to oxygen in air as much as possible, and which can be transported and stored with an industrial and economical advantage, as well as a method of preparing such a magnetic paint.
As a result of an earnest study of the present inventors, it has been that found by kneading magnetic particles such as fine magnetic iron based alloy particles, acicular fine magnetic iron oxide particles and plate-like fine magnetic ferrite particle having an average particle size of not greater than 0.25 .mu.m, a binder resin and an organic solvent by using a twin-shaft continuous kneader comprising a container and two stirring shafts disposed and rotatably journaled in parallel with each other in the container, in which each of the stirring shaft has alternately screw portions and paddle portions mounted to the stirring shaft, the ratio of the shaft length to the shaft diameter of the stirring shaft is not less than 25 and a clearance between the wall of the container and the end of the paddle end is not greater than 0.25 mm, and if required, adding a solvent to the kneaded product and diluting them under kneading by using dilution kneader, the thus-obtained magnetic paint is a composition kneaded such that the-solid content of the magnetic particles and the binder resin in the kneaded composition is from 65 to 85% by weight, and the binder resin is from 5 to 30% by weight based on the magnetic particles, and shows a high dispersibility so that a gloss at 45.degree. after dispersion for 6 hours is not less than 120% when formed into a coating film, and the magnetic paint can be packaged under vacuum with a synthetic resin film having acid resistance, water proofness and solvent resistance. The present invention has been attained on the basis of this findings.